U.S. patent application number 13/878840 was filed with the patent office on 2013-09-26 for voltage control in a vehicle electrical system.
The applicant listed for this patent is Rasmus Rettig, Werner Schiemann. Invention is credited to Rasmus Rettig, Werner Schiemann.
Application Number | 20130249285 13/878840 |
Document ID | / |
Family ID | 44645702 |
Filed Date | 2013-09-26 |
United States Patent
Application |
20130249285 |
Kind Code |
A1 |
Rettig; Rasmus ; et
al. |
September 26, 2013 |
VOLTAGE CONTROL IN A VEHICLE ELECTRICAL SYSTEM
Abstract
A method for maintaining a predetermined voltage in a
battery-supported vehicle electrical system during the operation of
an electrical starter includes steps of operating the electrical
starter on the vehicle electrical system during a first time phase
in series with a limiting resistor, in order to limit the current
flowing through the starter, and operating the electrical starter
on the vehicle electrical system during a second time phase with a
reduced limiting resistance, in order to increase a power converted
by the starter. A transition from the first time phase to the
second time phase is controlled on the basis of electrical
parameters sampled during the first time phase at the limiting
resistor.
Inventors: |
Rettig; Rasmus; (Hamburg,
DE) ; Schiemann; Werner; (Fellbach, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Rettig; Rasmus
Schiemann; Werner |
Hamburg
Fellbach |
|
DE
DE |
|
|
Family ID: |
44645702 |
Appl. No.: |
13/878840 |
Filed: |
September 9, 2011 |
PCT Filed: |
September 9, 2011 |
PCT NO: |
PCT/EP2011/065677 |
371 Date: |
June 14, 2013 |
Current U.S.
Class: |
307/10.6 |
Current CPC
Class: |
F02N 2250/02 20130101;
H02P 1/04 20130101; F02N 11/08 20130101; F02N 2200/044
20130101 |
Class at
Publication: |
307/10.6 |
International
Class: |
F02N 11/08 20060101
F02N011/08 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 13, 2010 |
DE |
10 2010 042 396.3 |
Claims
1-9. (canceled)
10. A method for maintaining a predetermined vehicle system voltage
in a battery-supported vehicle electrical system during the
operation of an electrical starter which is included in the vehicle
electrical system, comprising: operating the electrical starter on
the vehicle electrical system during a first phase in series with a
first limiting resistance of a limiting resistor, to limit the
current flowing through the starter; and operating the electrical
starter on the vehicle electrical system during a second phase with
a second limiting resistance of the limiting resistor lower than
the first limiting resistance, in order to increase a power
converted by the starter; wherein a transition from the first phase
to the second phase is controlled on the basis of electrical
parameters sampled during the first phase at the limiting
resistor.
11. The method as recited in claim 10, further comprising:
determining a threshold value during the first phase; and
initiating the second phase when a current flowing through the
limiting resistor falls below a predetermined threshold value.
12. The method as recited in claim 11, wherein the threshold value
is determined as a value which is no greater than I 1 R K + R 2 R K
+ R 1 ; ##EQU00006## I.sub.1 being the current flowing during the
first phase; R.sub.K(being the sum of ohmic resistances in a
circuit which includes a battery of the vehicle electrical system,
a first line from the battery to the limiting resistor, a second
line from the limiting resistor to the starter, and the starter;
R.sub.1 being the first limiting resistance; and R.sub.2 being the
second limiting resistance.
13. The method as recited in claim 12, wherein R.sub.K is
determined as follows: determining voltages at both terminals of
the limiting resistor at a first point in time; determining the sum
of the resistances of the battery and the first line leading from
the battery to the limiting resistor on the basis of the voltages
determined at the first point in time; determining voltages at both
terminals of the limiting resistor at second and third points in
time which are different from one another; and determining the sum
of the resistances of the second line leading from the limiting
resistor to the starter and of the starter on the basis of the
voltages determined at the second and third points in time.
14. The method as recited in claim 13, wherein, at the first point
in time, inductive influences in the area of the battery and the
first line have decayed and a rotational speed of the starter is so
low that an electromotive counter force of the starter is
negligibly small.
15. The method as recited in claim 13, wherein the third point in
time is so far beyond the second point in time that currents
flowing through the limiting resistor differ significantly from one
another at the second and third points in time.
16. The method as recited in claim 13, wherein at least a fourth
voltage is determined at a fourth point in time at the two
terminals of the limiting resistor, and the determination of the
sum of the resistances of the second line leading from the limiting
resistor to the starter and of the starter takes place on the basis
of the voltages determined at the second, third, and fourth points
in time.
17. The method as recited in claim 10, further comprising:
determining a triggering point in time on the basis of voltage
measurements at the limiting resistor during the first phase; and
initiating the second phase when the determined triggering point in
time is reached.
18. A device for maintaining a predetermined voltage in a
battery-supported vehicle electrical system during the operation of
an electrical starter which is included in the vehicle electrical
system, comprising: a controllable limiting resistor for operating
the starter on the vehicle electrical system in series with a
limiting resistance of the limiting resistor; a first sampling unit
and a second sampling unit for recording voltages at different
terminals of the limiting resistor; a timer; a control unit which
selectively varies the limiting resistance, wherein: the electrical
starter is operated on the vehicle electrical system during a first
phase in series with a first limiting resistance of the limiting
resistor, to limit the current flowing through the starter; and the
electrical starter is operated on the vehicle electrical system
during a second phase with a second limiting resistance of the
limiting resistor lower than the first limiting resistance, in
order to increase a power converted by the starter; and a
transition from the first phase to the second phase is controlled
on the basis of electrical parameters sampled during the first
phase at the limiting resistor.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method and a device for
maintaining a predetermined voltage in a battery-supported vehicle
electrical system.
[0003] 2. Description of the Related Art
[0004] An internal combustion engine of a motor vehicle is
generally started with the aid of an electrical starter. The power
required for operating the starter is withdrawn from a battery of
the vehicle electrical system of the motor vehicle. A vehicle
system voltage may drop so much due to the load of the battery by
the starter during the starting procedure that other consumers in
the vehicle electrical system are not supplied with sufficient
voltage. In particular, a consumer controlled by a microprocessor
may react sensitively to an excessively low vehicle system voltage
and may not function or may only function restrictedly for a
predetermined time span. The consumer may be a control unit which
is relevant for the operation and/or the safety of the motor
vehicle.
[0005] If the motor vehicle is equipped with a start-stop
mechanism, which already shuts down the internal combustion engine
during a short stop and only restarts it when needed, starting
procedures may be relatively frequent during travel of the motor
vehicle and temporary failures of a consumer may be particularly
serious. The present invention is therefore based on the object of
providing a method and a device for ensuring a predetermined
vehicle system voltage during a starting procedure.
BRIEF SUMMARY OF THE INVENTION
[0006] A method according to the present invention for maintaining
a predetermined voltage in a battery-supported vehicle electrical
system during the operation of an electrical starter includes steps
of operating the electrical starter on the vehicle electrical
system during a first time phase in series with a limiting
resistor, in order to limit the current flowing through the
starter, and operating the electrical starter on the vehicle
electrical system during a second time phase with a reduced
limiting resistance, in order to increase a power converted by the
starter. A transition from the first time phase to the second time
phase is controlled on the basis of electrical parameters sampled
at the limiting resistor during the first time phase.
[0007] In this way, it may be ensured that the reduction of the
limiting resistance occurs at an optimized point in time, i.e., as
early as possible to maximize the power converted by the starter
and thus to accelerate the starting procedure, and simultaneously
as late as necessary, to avoid a drop of the battery voltage below
a predetermined value. Through the control as a function of values
detected individually during each starting procedure, variable
influencing variables may be taken into consideration, such as a
mechanical resistance of the internal combustion engine at
different temperatures, aging and temperature influences on the
battery, and a power withdrawal by further consumers.
[0008] During the first phase, a threshold value may be determined
and the second phase may be initiated when the current flowing
through the limiting resistor falls below the determined threshold
value. The initiation of the second phase or the reduction of the
limiting resistance may thus be carried out on the basis of a
relatively simple determination.
[0009] The determination of the threshold value is advantageously
carried out on the basis of measurements, which are collected at
predetermined points in time, in order to minimize both inductive
influences and also a variable counter electromotive force of the
starter during the measuring period of time. The points in time are
based on inductances of lines and the speed of the increase of the
counter electromotive force during the starting procedure.
[0010] The method may be carried out on the basis of a plurality of
measurements which are rectified with respect to time, in order to
determine the optimized changeover point in time first coarsely and
then more and more finely.
[0011] In an alternative specific embodiment for the determination
and monitoring of a threshold value for the current, the method may
include steps of determining a point in time on the basis of
voltage measurements at the limiting resistor during the first
phase; and initiating the second phase when the determined point in
time is reached. This determination may be carried out on the basis
of differential equations and may allow a prognosis of the
optimized changeover point in time, without having to monitor the
current flowing through the starter.
[0012] A device according to the present invention for maintaining
a predetermined voltage in a battery-supported vehicle electrical
system during the operation of an electrical starter includes a
controllable limiting resistor for operating the starter on the
vehicle electrical system in series with the resistor, a first
sampling unit and a second sampling unit for recording voltages at
different terminals of the resistor, a timer, and a control unit
for reducing the limiting resistance in accordance with the
above-described method.
[0013] The device allows the optimized transition from the first
phase into the second phase, without requiring sensors, which are
to be attached at a distance to the limiting resistor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 shows a circuit diagram of a vehicle electrical
system in a motor vehicle.
[0015] FIG. 2 shows a current controller for use in the vehicle
electrical system of FIG. 1.
[0016] FIG. 3 shows curves of voltages and currents in the vehicle
electrical system of FIG. 1.
[0017] FIG. 4 shows a flow chart of a method on board the motor
vehicle of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0018] FIG. 1 shows a circuit diagram of a vehicle electrical
system 100 in a motor vehicle 102. Vehicle electrical system 100
includes a battery 105, a limiting resistor 110, a starter 115, and
a first line 120 from battery 105 to limiting resistor 110 and a
second line 125 from limiting resistor 110 to starter 115. A
starter relay 130 is incorporated in second line 125. Furthermore,
a consumer 135, which may be representative for a number of
different consumers, is connected in parallel to battery 105.
[0019] Consumer 135 is operated in vehicle electrical system 100
and requires the provision of an operating voltage for this
purpose, which may not fall below a predetermined threshold value.
For example, consumer 135 may include an electronic or
microprocessor-controlled component, which resets (executes a
reset) after the supply voltage falls below the threshold value,
and then requires a certain reset time to be ready for use again.
It is the object of the present invention to prevent a voltage drop
at battery 105 below this threshold value.
[0020] Motor vehicle 102 includes an internal combustion engine
(not shown), which may be started with the aid of starter 115. When
internal combustion engine is running, it drives a generator (also
not shown), which charges battery 105. During a starting procedure
of the internal combustion engine, however, the generator is not in
operation or only causes a negligible charge of battery 105.
[0021] In order to start the internal combustion engine, starter
relay 130 is closed, so that starter 115 is connected essentially
in parallel to battery 105. Starter 115 includes a high-performance
DC electric motor, which then begins to rotate at increasing speed.
The electric motor drives the internal combustion engine until it
starts. At the latest when the internal combustion engine is
running, starter relay 130 is opened and starter 115 gradually
comes to a standstill. A starting procedure generally last several
seconds, although only a short first phase of the starting
procedure is considered hereafter.
[0022] Immediately after the closing of starter relay 130,
stationary starter 115 has a counter EMF close to zero, so that a
large current flows in a circuit 140 from battery 120 through first
line 120, limiting resistor 110, second line 125, starter relay
130, and starter 115. Since battery 105 has an internal resistance
not equal to zero, the vehicle system voltage available at its
terminals drops as a function of the flowing current, so that the
risk exists that consumer 135 will reset.
[0023] It is known that the drop of the vehicle system voltage may
be reduced in that limiting resistance 110 is set to a
predetermined value greater than 0.OMEGA.. In order to maximize the
power converted by starter 115 for the starting procedure, limiting
resistance 110 may be reduced after a predetermined time. A point
in time at which limiting resistance 110 may be reduced without the
vehicle system voltage falling below the predetermined threshold
value is generally determined once within the scope of a conception
of vehicle electrical system 100 and is no longer changed
thereafter. A reduction of limiting resistance 110 on the basis of
measurements in the area of battery 105 requires measuring units,
which are to be avoided as much as possible for reasons of
cost.
[0024] If limiting resistance 110 is reduced excessively early or
excessively strongly, the risk of collapse of the vehicle system
voltage exists, so that consumer 135 resets. If limiting resistance
110 is reduced excessively late or excessively little, however, the
power converted by starter 115 is not maximized, so that the
starting procedure of the internal combustion engine may be
impaired or lengthened.
[0025] In order to observe the occurring sequences in detail during
a starting procedure more precisely, battery 105, limiting resistor
110, starter 115, and lines 120 and 125 are shown as replacement
circuit diagrams in FIG. 1, which include ohmic resistors,
inductors, and a voltage source.
[0026] Battery 105 provides a voltage Ubatt; an internal resistance
of battery 105 is Ri. First line 120 includes an internal
resistance Rzul1 and an inductance Lzul1. Limiting resistance 110
is R, the limiting resistance being variable, for example, using a
semiconductor or an electromechanically controlled switch, between
two or more values R1, R2, . . . , and R1>R2> . . . . Last
used resistance value Ri may be zero or approximately zero. Second
line 125 includes an internal resistance Rzul2 and an inductance
Lzul2. Starter 115 includes an inductance L_St, an internal
resistance R_St, and a voltage source G_EMK, whose provided voltage
corresponds to the counter induced voltage (counter electromotive
force, counter EMF) of the starter and is dependent on the
rotational speed of the starter and counteracts battery voltage
U_batt.
[0027] Starter relay 130 is assumed to be ideal or its electrical
resistance in the closed state is modeled by second line 125.
Connections which go to ground are also assumed to be ideal or
modeled in remaining circuit 140.
[0028] The following values are assumed as examples in vehicle
electrical system 100:
[0029] Ri 6 m.OMEGA.
[0030] Rzul 2 m.OMEGA.
[0031] Lzul1 5 uH
[0032] Rzul2 5 m.OMEGA.
[0033] Lzul2 1 uH
[0034] L_St 4.3 uH
[0035] R_St 1.75 m.OMEGA.
[0036] G_EMK time-dependent, see text
[0037] Voltmeters with indications of voltages are shown in FIG. 1.
The voltmeters indicate at which point in vehicle electrical system
100 which voltage is applied. This is accordingly true for an
ammeter in the area of starter relay 130.
[0038] FIG. 2 shows an integrated current controller 200 for use in
the place of limiting resistor 110 in FIG. 1. Current controller
200 includes a first terminal 205 for connection to first line 120
and a second terminal 210 for connection to second line 125. A
limiting resistance 215, which may be selectively reduced to zero
with the aid of a controllable switch 220, is situated between
terminals 205 and 210. Voltages between terminals 205 and 210 and
ground are recorded with the aid of voltmeters 225 and 230,
respectively, and provided to a control unit 235. Control unit 235
is connected to a timer 240 and configured for the purpose of
controlling switch 220. Timer 240 is configured for the purpose of
providing a time normal for determining times or time intervals
between events, in particular measurement points in time for
voltmeters 225 and 230.
[0039] In the illustration of FIG. 2, limiting resistance 215 or
the zero resistance lies between terminals 205 and 210 depending on
the position of switch 220. In other specific embodiments, multiple
limiting resistors 215 and optionally also multiple switches 220
may be provided, to set multiple different electrical resistances
between terminals 205 and 210. In one specific embodiment, the
resistor lying between terminals 205 and 210 is implemented by a
semiconductor arrangement, which is preferably continuously
controlled by control unit 235.
[0040] FIG. 3 shows curves of voltages and currents in the vehicle
electrical system from FIG. 1. A time is plotted in a horizontal
direction and curves 310 through 370 of a current or various
voltages are plotted in the vertical direction.
[0041] First curve 310 describes current I_Starter through starter
115. As is apparent from FIG. 1, the same current I_Starter flows
through all branches of circuit 140 at every point in time, since
this is a pure series circuit. At point in time 0, the starting
procedure is initiated, in that starter relay 130 is closed.
I_Starter increases rapidly to almost 600 A and drops therefrom in
accordance with a logarithmic function. A variable mechanical
resistor of the internal combustion engine may result in a
superimposed periodic variation of starter current I_Starter and is
not considered in the present consideration. At point in time a at
approximately 130 ms, limiting resistance 110 is reduced to 0.
I_Starter subsequently increases again, until point in time b at
approximately 140 ms, to almost 600 A and again drops
logarithmically. Current I_Starter flowing through starter 115 is
primarily dependent on the rotational speed of starter 115. With
increasing rotational speed, the voltage drop at starter 115 also
increases and the current flowing through starter 115
decreases.
[0042] Lowermost curve 370 describes voltage U_Sternpkt at battery
105, which is also the supply voltage of consumer 135. At the
beginning of the starting procedure, U_Sternpkt is 12 V and then
drops rapidly to approximately 8.4 V. U_Sternpkt subsequently
increases essentially inversely proportionally to I_Starter and
reaches approximately 9.2 V at point in time a. Between points in
time a and b, U_Sternpkt decreases again to approximately 8.4 V and
then increases again essentially inversely proportionally to
I_Starter.
[0043] Second curve 320 from the top in FIG. 3 describes counter
EMF G_EMK, which arises during operation of starter 115 and is
dependent on the rotational speed of starter 115. G_EMK increases
logarithmically to approximately 8 V, the increase being uniform
enough to be considered to be linear in a sufficiently short
phase.
[0044] Third curve 320 from the top in FIG. 3 describes input
voltage U_in, which is measurable in relation to ground at a point
between first line 120 and limiting resistor 110. U_in is the
voltage which voltmeter 225 samples in FIG. 2. Sixth curve 360 from
the top in FIG. 3 describes output voltage U_out, which is
measurable in relation to ground at a point between first line 120
and limiting resistor 110. U_out is the voltage which voltmeter 230
samples in FIG. 2.
[0045] U_in and U_out qualitatively follow U_Sternpkt, in that they
drop rapidly at point in time 0 from a value close to the no-load
voltage of battery 105 and then increase logarithmically until
point in time a, drop again until point in time b and then again
increase logarithmically.
[0046] Fourth curve 340 from the top in FIG. 3 describes first
induced voltage U_L_ind1 at inductance L_zul1 of first line 120 in
FIG. 1. In a corresponding way, fifth curve 350 from the top in
FIG. 3 describes second induced voltage U_L_ind2 at inductance
L_zul2 of first line 120 in FIG. 1. Both induced voltages U_L_ind
are only positive in each case in a short time phase after a change
of current I_Starter. These time phases are proportional to the
absolute values of inductances Lzul1 and Lzul2 and resistances
Rzul1 and Rzul2, respectively, corresponding thereto. The time
phases each begin at points in time a and b.
[0047] Induced voltages U_L ind1 at inductance L_zul1 and U_L_zul2
at inductance L_zul2 decay enough within a comparatively short time
that they are negligibly small in relation to the other voltages of
circuit 140. For the assumed values of the elements in FIG. 1, this
is the case after approximately 2-4 ms. After decay of the
inductance effects, U_L_zul1, U_L_zul2, and U_L_St are each
approximately 0, so that L_zul1, L_zul2, and L_St are no longer
taken into consideration.
[0048] It is explained hereafter how, on the basis of measurements
at limiting resistor 110 during the first phase, i.e., while
limiting resistance 110 assumes a first predetermined value R1
greater than zero, a threshold value I_switch may be determined,
which current I_Starter must fall below before limiting resistance
110 may be reduced to a second predetermined value R2, without
voltage U_Sternpkt dropping by more than a predetermined extent.
Specified times are determined from the closing of starter relay
130 at point in time 0. If limiting resistance 110 is to be reduced
step-by-step in more than two phases, the determination specified
hereafter may thus accordingly be carried out iteratively.
[0049] Current I_Starter flowing through circuit 140 may be
determined as follows by measurements of U_in and U_out at limiting
resistor 110:
I starter = U out - U in R 1 . ##EQU00001##
[0050] Circuit 140 may be divided into an input circuit, which is
composed of battery 105 and first line 120, and an output circuit,
which is composed of limiting resistor 110, second line 125, and
starter 115.
[0051] Before counter EMF G_EMK in the output circuit becomes
noticeably large, in that it reaches a value greater than
approximately 100 mV, which occurs approximately at the point in
time 4 ms in the case of the above-specified values, the output
circuit is determined as:
U.sub.out=(R.sub.zul2+R.sub.St)I.sub.Starter.
[0052] In this case:
I starter = U out - U in R 1 ##EQU00002##
[0053] After sufficient decay of second induced voltage U_L_Ind2,
in the case of the above-assumed values approximately 2.5 ms after
the beginning of the starting procedure, the input circuit is
determined at two successive points in time as:
U.sub.Batt=I.sub.1(R.sub.i+R.sub.zul1)+U.sub.1in
and:
U.sub.Batt=I.sub.2(R.sub.i+R.sub.zul1 )+U.sub.2in.
[0054] First measurement U_1in may coincide with the measurement of
U_in, which is carried out to determine the output circuit. Second
measurement U_2in takes place at the greatest possible time
interval from the first measurement, to increase the precision of
the method, but at the same time early enough so it does not lie
after the optimized changeover point in time. In the case of the
above-specified values, a time interval of approximately 5-50 ms
between the measurements U_1in and U_2in is meaningful, preferably
15-30 ms, more preferably approximately 20 ms.
[0055] When subtracted from one another, the last two formulas
result in:
(I.sub.2-I.sub.1)(R.sub.i+R.sub.zul1)=U.sub.1in-U.sub.2in
Furthermore:
U.sub.Sternpkt=U.sub.Batt-IR1 or
I=(U.sub.batt-U.sub.Sternpkt)/R.sub.i
[0056] Ri may be dependent on employed battery 105 or a battery
type as the maximum value.
[0057] In summary:
U.sub.batt=I(R.sub.i+R.sub.zul1+R1+R.sub.zul2+R.sub.St)+U.sub.EMK
[0058] If two measurements of U_in or U_out are carried out at a
short time interval (see above), so that counter EMF G_EMK between
the measurements may be considered to be constant, the following
applies:
I.sub.1(R.sub.i+R.sub.zul1+R1+R.sub.zul2+R.sub.St)=I.sub.2(R.sub.i+R.sub-
.zul1+R2+R.sub.zul2+R.sub.St)
[0059] The current through starter 115 in the second phase, after
the reduction of limiting resistance 110 from R1 to R2, is thus
assessed as:
I.sub.2=I(R.sub.i+R.sub.zul1+R1+R.sub.zul2+R.sub.St)/(R.sub.i+R.sub.zul1-
+R2+R.sub.zul2+R.sub.St).
[0060] At the optimized point in time of the reduction,
therefore:
I decrease = ( U batt - U Sternpkt R i ) ( R i + R zul 1 + R 2 + R
zul 2 + R St ) ( R i + R zul 1 + R 1 + R zul 2 + R St ) .
##EQU00003##
[0061] As already stated, in the above formulas, terms
R.sub.i-R.sub.zul1 may each be replaced by
U 1 in - U 2 in I 2 - I 1 . ##EQU00004##
[0062] Term U_batt -U.sub.Sternpkt specifies the absolute value by
which battery voltage U_batt may drop at most upon closing of
starter relay 130. Ohmic resistance R_zul1 of first line 120 may be
determined once, for example, within the scope of a conception of
motor vehicle 102.
[0063] On the basis of measurements of voltages U_in or U_out at
limiting resistor 110 at predetermined points of time, current
I_decrease may thus be determined, which flowing current I must
fall below before limiting resistance 110 is reduced from R1 to
R2.
[0064] Instantaneously flowing current I may be determined
continuously or periodically with the aid of measurements of U_in
and U_out and it may be checked whether the following applies:
I 1 < I decrease .revreaction. U out - U in R 1 < I decrease
. ##EQU00005##
[0065] In the described way, the reduction of limiting resistance
110 from R1 to R2 may be carried out or signaled at the optimum
point in time simply and reliably, i.e., as early as possible
without neutral point displacement voltage U_Sternpkt dropping more
than predefined by U.sub.batt-U.sub.Sternpkt. Limiting resistance
110 may be reduced to a positive value or to zero. A further
step-by-step reduction of limiting resistance 110 in a
corresponding way is possible. In still another specific
embodiment, the reduction may also take place continuously. Device
200 in FIG. 2 is configured to carry out this method.
[0066] FIG. 4 shows a flow chart of a method on board motor vehicle
102 of FIG. 1. In a first step 405, starter 115 in vehicle
electrical system 100 of motor vehicle 102 is operated in series
with limiting resistor 110.
[0067] In a following step 410, voltage values U_in and U_out are
determined at limiting resistor 110. For this purpose, it is
awaited until inductive effects in circuit 140 have decayed. U_out
is then determined, on the basis of which resistances are
determined in the output circuit, which is composed of limiting
resistor 110, second line 125, and starter 115. To determine
resistances in the input circuit, which is composed of battery 105
and first line 120, two measurements of U_in are also carried out
at a predetermined time interval after the decay of the inductive
effects in circuit 140. The first of these measurements may be
carried out simultaneously with the determination of U_out.
[0068] On the basis of the measurements, in a first step 415, a
threshold value for the current flowing through limiting resistor
110 is determined. Subsequently, in a step 420, the current flowing
through limiting resistor 110 is determined continuously on the
basis of simultaneous determinations of U_in and U_out, for which
purpose the difference of U_in and U_out is divided by the
resistance value of limiting resistance 110.
[0069] If the determined current falls below the previously
determined threshold value, in a step 425, limiting resistance 110
is reduced and starter 115 is operated in series with reduced
limiting resistance 110 at battery 105. Due to the way in which
threshold value is determined in step 415, it is ensured that
voltage U_Sternpkt applied to battery 105 does not drop below a
predetermined voltage upon the reduction of limiting resistance
110. Method 400 is then terminated.
* * * * *